There is no way to set/unset a Flag without losing its span information

[LikeLakers2]

Hi! I was doing some derive-macro practice over at https://github.com/LikeLakers2/michis_rand_distr_derive.

In the midst of this, I found myself wanting to set a Flag value - but found there was no way to do so without losing its span information.

In my specific case, I have two options that can be applied to enum variants - a weight (how likely that variant is to be selected) and a skip option (if specified, the variant will never be selected). What I wanted to do was, if the weight was a zero-like value, erase the weight value and enable the skip flag. Unfortunately, I found there was no way to do this with Flag alone.

I did find a solution, SpannedValue<Flag>, which allows me to recreate the Flag value without (technically) losing the span information. However, this feels hacky, as now I'm storing the Span twice - and more importantly, I am forced to add #[darling(default)] to that field, because SpannedValue<Flag> does not inherit Flag's property of being inherently optional.

I'm hoping something can be done to handle this.

(P.S. If you're curious about the relevant code: https://github.com/LikeLakers2/michis_rand_distr_derive/blob/98200c0ed81ddace432787e4fe4214db6085049e/src/standard_distribution/derive_variant.rs#L19-L27)

[TedDriggs]

Instead of modifying the skip field in check_and_correct if the weight evaluates to 0, could is_skipped check whether the flag is present or the weight is zero?

It looks like you are checking skip.is_present elsewhere in the code, but could instead call is_skipped in those places?

[LikeLakers2]

Hmm... that would probably work. Thank you for the suggestion.

That said, I still wonder if it may be useful to allow (un)setting a Flag. What do you think?

[TedDriggs]

That can be achieved by setting the field to be Flag::default(). Why would you need a method to do that?

[TedDriggs]

Check out the impl of Flag; you can't unset it and keep the span. https://github.com/TedDriggs/darling/blob/b7a248f8ee4503a949de0730170934587dccfb8e/core/src/util/flag.rs#L45

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________________________________ From: MichiRecRoom ***@***.***> Sent: Friday, January 17, 2025 8:09:19 PM To: TedDriggs/darling ***@***.***> Cc: Ted Driggs ***@***.***>; Comment ***@***.***> Subject: Re: [TedDriggs/darling] There is no way to set/unset a `Flag` without losing its span information (Issue #315) Why would you need a method to do that? Sorry, I neglected to mention that I would want to keep the span info. That's why I'd want a method to do that. — Reply to this email directly, view it on GitHub<#315 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ABEVHR5KCIZJ76GO4VPNVL32LHHW7AVCNFSM6AAAAABTA3N2HOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDKOJZGUYTKNRQGY>. You are receiving this because you commented.Message ID: ***@***.***>

[LikeLakers2]

@TedDriggs Sorry, let me clarify.

While your suggestion to change how is_skipped is implemented would work for me... I still think it would be useful for Flag to have methods to set/unset whether it's present, without losing the span information.

Flag::default() won't work for this because it won't preserve the span info. My original solution, SpannedValue<Flag> would work - but then we're duplicating the span info.

I would much prefer a method on Flag, let's say set_present(bool), which sets whether the Flag is considered present - without changing the span info.

I hope that helps you understand.

[TedDriggs]

The struct literally cannot be unset while preserving its span; internally it's just Option<Span>.

[LikeLakers2]

I'm aware that it's just an Option<Span> internally. I get why it's like that.

But I also don't care how it works internally - I'm a user of darling, and all I care is that it works as I expect.

If implementing methods to set/unset a Flag while preserving its span info requires changing how it works internally (maybe to a (bool, Span)), then so be it.

If you don't want to do that, then just say so.